1. Technical Field
The present invention relates to a tuning fork vibration device having a tuning fork vibration section formed on a SOI substrate which is driven by vibration of a piezoelectric layer.
2. Related Art
The development of communication devices such as clocks, personal computers and the like is trending toward a reduction in size and a reduction in power consumption; and for this reason, a reduction in size and a reduction in power consumption of clock modules are demanded. As a vibration device that is used in an oscillator section of a clock module, a 32 kHz tuning fork vibration device is still used in order to take advantage of its conventional design resource and power saving property. The tuning fork vibration device has a structure in which a piezoelectric such as quartz that is processed in a tuning fork configuration is interposed between electrodes in a manner to be driven, and has various advantages such as excellent temperature characteristics and excellent power saving property. However, in the case of a 32 kHz tuning fork vibration device, the arm length of the tuning fork is several mm, and the overall length including its packaging amounts to about 10 mm.
In recent years, vibration devices that use a piezoelectric thin film formed on a silicon substrate, instead of quartz, have been developed. Such vibration devices have a laminate structure in which a piezoelectric thin film is interposed between upper and lower electrodes, and generate flexural vibration due to expansion and contraction movements in a plane. As the structure of such vibration devices, a beam type structure (see FIG. 1 of Japanese Laid-open patent application JP-A-2005-291858) and a structure in which a tuning fork vibration device is formed with two beams (see FIG. 1 of Japanese Laid-open patent application JP-A-2005-249395) are known.
It is noted that, in the vibration devices using a piezoelectric thin film formed on a silicon substrate, the silicon substrate can only be made to a thickness of about 100 μm at most, such that the sound speed of flexural vibration can be lowered only to about several 100 m/s. In order to obtain a resonance frequency at several tens kHz band, the arm-length of the beam needs to be several mm or greater, which causes a problem in that miniaturization of the clock module is difficult.